Parsing a JSON query string into a JSON object

Question

The query on the API is structured like this:

../myapi/products?_q=genre,retail_price&_a=g,p&p.gt=10&p.lt=20&g.eq=POP

There are two arrays: _q which lists the query parameters and _a listing the corresponding aliases. So p -> retail_price and g -> genres

I can parse this into:

{$and : [ genre: { '$eq': 'POP' }, retail_price: { '$gt': '10' }, retail_price: { '$lt': '20' } ]}

Almost happy. But there are a couple of problems with this approach: 1. the '$eq' etc instead of $eq etc 2. the numeric value is now a string '10'

I consider (2) to be a nasty one. Since the server cannot know the type (maybe it should be '10' instead of 10).

So, I want to try another approach. And that is parsing it all into a queryString and then convert that with JSON.parse()

First I put up some query string and try it in the shell:

db.products.find({$and: [{genre: { $eq: 'ROC'}}, {retail_price: {$gt:7}}, {retail_price: {$lt:10}}]})

Works like a charm.

Then I tried this:

var queryStr = "{$and: [{genre: { $eq: 'ROC'}}, {retail_price: {$gt:7}}, {retail_price: {$lt:10}}]}";

and: (Product is a mongoose model)

Product.find(JSON.parse(queryStr), function(err, product) {
    if (err)
        res.send(err);

        res.json(product);
    });

To my surprise it did not work at all.

Also doing

console.log(JSON.stringify(JSON.parse(queryStr)));

Does not write output to the console.

What is happening here?

Jason Cust · Accepted Answer · 2015-05-05 11:42:11Z

1

The first thing is queryStr is not valid JSON. What you have is a string that looks like an object. But you might ask "isn't that JSON?". Short answer: no. Long answer: since JSON is a lightweight data exchange format it has to be readable by a variety of languages (not just Javascript). So quoting keys is a requirement to make this possible:

var json = '{"foo": true, "bar": 123}';
var str = '{foo: true, bar: 123}';
console.log(JSON.parse(json)); // Object {foo: true, bar: 123}
console.log(JSON.parse(str)); // SyntaxError: Unexpected token f

So, you could stringify the query object to JSON and then parse it before passing it to your mongoose method:

// On the client
var queryJSON = JSON.stringify({$and: [{genre: { $eq: 'ROC'}}, {retail_price: {$gt:7}}, {retail_price: {$lt:10}}]});
// On the server
var query = JSON.parse(queryJSON); // Object

That said, back to your original two concerns:

Quoted key names: they have zero impact in this regard so they shouldn't be a concern at all.
Incorrect value types: it looks like you already have a procedure for formatting the query to the correct object so you could use Number to coerce the string to a number value. (e.g. Number('10') // 10)

answered May 5, 2015 at 11:42

Jason Cust

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2 Comments

dexter Over a year ago

Super! I can now indeed do what I wanted. Actually to let the server know that a number is indeed a number I've added a little syntax (e.g. p.eg=(10), instead of p.eq=10). It now works like a charm. I can now write stuff like: ../products?_q=genre,filling_rate,retail_price&_a=g,f,p&g.eq=ROC&f.eq=(95)&p.lt=(8)

dexter Over a year ago

I should add, that when using Mongoose with a schema, the addition of (..) is not needed.

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